1. Field of the Invention
This invention relates to a photoelectric conversion apparatus which includes a plurality of photoelectric conversion cells each including a transistor having a control electrode area, wherein the potential of the control electrode area of each transistor is controlled to store carriers produced by optical pumping in the control electrode area, to read the stored voltage, and to nullify the stored carriers, and more particularly to a photoelectric conversion apparatus which is intended to perform precise peak detection.
2. Related Background Art
FIG. 1A is a schematic plan view of one example of a photoelectric conversion cell described in Japanese patent application No. 252653/1985, FIG. 1B is a cross sectional view taken along the line A--A of FIG. 1A and FIG. 1C is an equivalent circuit diagram of the cell.
In these Figures, an n-silicon substrate 1 has an n.sup.- -epitaxial layer 3 formed thereon which has a p-base area 4 formed thereon which has n.sup.+ -emitter areas 5 and 5' formed thereon. The emitter areas 5 and 5' are connected to emitter electrodes 8 and 8', respectively.
In this example an insulating area 14 and the underlying n.sup.+ -area 15 constitutes a device separating area 2 which separates adjacent photoelectric conversion cells from each other
Formed on p-base area 4 is an oxide film 6 on which is formed a capacitor electrode 7 with an insulating film 16 thereon. Formed on film 16 is a light shielding film 17 which shields light from the area on which the capacitor electrode and emitter electrode are formed with a photosensitive surface being formed in the main portion of p-base area 4. A protective insulating film 18 is formed o the light shielding film 17 and the insulating film 16 portion constituting the light-sensitive face. FIG. 1B also shows an n.sup.+ layer 11 underlying layer 1, with an electrode 12 underlying layer 11.
In the basic operation, first assume that the p-base area 4 which is the base of a bipolar transistor is in an initial negative-potential state. When light enters the photosensitive face of this p-base area 4, electron-positive hole pairs will be produced, the positive holes of which are stored in p-base area 4, which changes the potential of p-base area 4 in the positive-going sense (storage operation).
Subsequently, a positive read voltage pulse is applied to capacitor electrode 7 and a read signal, namely, optical information corresponding to a change in the base potential during storage operation is outputted from emitter electrodes 8 and 8' in a floating state (read operation). At this time, the quantity of electric charge stored in p-base area 4 virtually does not decrease, so that non-destructive reading is possible.
In order to eliminate the positive holes stored in p-base area 4, emitter electrode 8 is grounded and a positive refresh pulse voltage is applied to capacitor electrode 7. This biases p-base area 4 forwardly relative to n.sup.+ -emitter areas 5 and 5' to thereby eliminate the positive holes stored. When the refresh pulse falls down, p-base area 4 returns to its initial negative-potential state (refresh operation). Thereafter, similarly, storage, read and refresh operations are repeated.
Such a double-emitter photoelectric conversion cell allows a signal to be read from either of both the emitters, so that the mean value or peak value can be easily taken using one signal and light measurement and/or peak value detection can be performed in parallel with the signal reading.
FIG. 2 is a circuit diagram showing one example of a photoelectric conversion apparatus using cells described in the above patent application No. 252653/1985. In FIG. 2, double-emitter photoelectric conversion cells S1-Sn are arranged in a line. Emitter electrodes 8 are connected to an output line 101 via vertical lines L1-Ln and transistors T1-Tn. Respective signals are read serially to signal output line 101, amplified by amplifiers and output outside as an output signal V0.
On the other hand, emitter electrodes 8' are connected to a common line 102, so that the peak values Vp of respective signals appear on the common line 102. The use of peak values Vp allows adjustment of the gain of the signal output amplifier and the durations of storage in the photoelectric conversion cells. Further, peak value detection is possible at the same time as reading from the emitter electrodes 8, so that the image pickup operation is speeded up.
However according to the conventional photoelectric conversion apparatus, noise components due to dark current in the photoelectric conversion cell are contained in the output signal, so that the apparatus has the problem that the peak value Vp does not correspond accurately to the peak signal in the photoelectric conversion cell.